1. Field of the Invention
The present invention relates to a coil for generating a homogeneous magnetic field in the interior space of a hollow cylindrical coil and more particularly to a magnet for generating a magnetic field for a nuclear magnetic resonance type computer tomogram imaging instrument (referred to as an MRI hereinafter).
2. Description of the Prior Art
A coil for generating a homogeneous magnetic field used for an MRI magnet is commonly composed of a plurality of cylindrical coils or ring coils having the inner diameter of approximately 1 meter in order to accommodate an object, that is, a human body in the coil. For obtaining an excellent tomogram which is of sufficient quality to diagnose the state of a human body, it is necessary to maintain the homogeneity of the magnetic field strength in the imaging space accommodating a human body at a level of 1.times.10.sup.-6. For satisfying the necessary strength and the homogeneity of the magnetic field, a method for constructing a coil for generating a homogeneous magnetic field has been known, in which ring coils arranged coaxially are combined and their positions and values of electric current to be flowed are selected in such a manner that the homogeneity of the magnetic field is optimized. However, if the flux of the homogeneous magnetic field generated by a coil for generating a homogeneous magnetic field having such a construction is leaked to the external space, it may cause the electronic devices located close to the magnetic field to malfunction. Further, the magnetic field due to the leakage flux could magnetize a ferro-magnetic substance located close to the MRI and this may distort the magnetic field so as to adversely affect the homogeneous magnetic field space. As a result, when the coil for generating a homogeneous magnetic field is used alone, if the magnetic field having high homogeneity is generated, the expected homogeneous magnetic field cannot be obtained in the environment in which the MRI is used. Hence, in order to prevent the spread of leakage fluxes to the outside, a magnetic shield is often provided to cover the coil for generating a homogeneous magnetic field. However, the method for absorbing leakage fluxes using a ferro-magnetic substance has a problem that since the strength of leakage fluxes is higher with higher strength of a homogeneous magnetic field, especially in the case of using a superconductive MRI, the weight of the magnetic substance shielding material is too large, and the weight may be over the permissive load of the chamber floor in which an MRI is located. Especially for reducing the leakage flux in the superconductive MRI, therefore, the method of cancelling the magnetic field due to the leakage fluxes using a superconductive coil has been employed.
This method does not use any magnetic substance and hardly offers the effects caused by the magnetic substances located close to an MRI on homogeneous magnetic field space, so that it is easy to perform a magnetic field analysis needed to design a coil for generating a homogeneous magnetic field. It means that the magnetic field generating coil equipped with a second superconductive coil for cancelling the leakage fluxes is a very effective coil for a superconductive MRI which is especially required to form a highly homogeneous magnetic field space. This method indicates that a superconductive coil serves to cancel the magnetic field leaked out of a homogeneous magnetic field generated by a coil for generating a homogeneous magnetic field used as a superconductive MRI magnet. And it is referred to as an active shield type superconductive MRI magnet (hereinafter, simply referred to as an active shield type).
The active shield type must meet the following two requirements.
(1) Forming a homogeneous magnetic field at given homogeneity within given space; and
(2) Lowering the magnetic flux leaked outside of a given range from an MRI to be equal to or less than a given value.
A homogeneous magnetic field of high homogeneity with only several PPMs level variation is required as described above. The strength of the magnetic field due to leakage flux must be as small as 5 G (0.5 mT) or less in the space located at a predetermined distance from an MRI in comparison with the uniform space magnetic field strength of about 1 T. Furthermore, under the state of meeting these two conditions, the following actual practical requirements must be considered.
(3) In order to reduce a closeness feeling of a patient, or anxiety of such a feeling when a patient gets into an MRI in checking himself or herself as much as possible, the length of a coil for generating a homogeneous magnetic field must be as short as possible.
(4) The diameter of a coil for generating a homogeneous magnetic field must be as small as possible due to some restraints such as the size of a room and the height of its ceiling provided when an MRI is installed in a non-specific room.
In order to achieve higher homogeneity and restrain the strength of magnetic field due to leakage flux more, the following two requirements in designing and manufacturing the coil must be also considered.
(5) Constructions and conditions settings must be such that a magnetic field analysis which is essential designing a coil for generating a homogeneous magnetic field.
(6) Constructions must be such that a coil is manufactured with accuracy needed to secure a homogeneous magnetic field.
The coil for generating a homogeneous magnetic field itself must be large enough to secure given homogeneous space. Since an MRI must be somewhat restricted in size to be accommodated in the defined space of a room, the smaller the device, the more universal and the less expensive it is. Thus, it is preferable that the smaller coil for generating a homogeneous magnetic field can secure larger homogeneous space. However, the coil for generating a homogeneous magnetic field must have an inner diameter size of at least 1 meter, because a human body must be inserted into the inner space of the coil. Additionally, a lot of other coils, such as a high frequency coil and a gradient coil, also are disposed into this inner space, and further because a space for thermal insulation is required to install the coil in a cryostat when a superconducting magnet is used. On the other hand, the size of homogeneous magnetic field space is relevant to the size of a human body. Usually, a sphere with about 40 cm diameter is required as homogeneous magnetic field space. Further, in case of a superconductive magnet, the generated magnetic field is 0.5 T to 2 T which is ten times as large as about 0.1 T of the normal conductive magnet, so that the homogeneity required by it is several PPMs, which is one digit higher than that required by the normal magnet.
The coil for generating a homogeneous magnetic field provided in such a manner that the larger homogeneous space with higher homogeneity is formed, can be constructed by increasing the number of coil rings composing the coil. In case of, however, employing the construction wherein the homogeneity corresponding to the increased number of coil rings is secured, the axial length is increased, so that a patient may feel more closeness as mentioned above as well as the manufacture of an MRI magnet costs more because of the increased number of coil rings. Moreover, as the axial length is increased, the ampere-turn must be increased so as to secure the same magnetic field strength as before. Since the increase of coil rings brings about the increase of materials to be used and capacitance of a D/C power source, it results in an increase of weight and cost. Therefore, it is disadvantageous to unduly increase the coil rings in number even if it results in providing superior homogeneity.
The active shield type magnet has been already disclosed in laid-open the Japanese Patent Application No. 60-98344 entitled "Nuclear Magnetic Resonance Apparatus", U.S. Pat. No. 4,587,504 entitled "Magnet Assembly for Use in NMR Apparatus" and laid-open Japanese Patent Application No. 60-217608 entitled "Homogeneous magnetic Field Coil". These publications, however, do not disclose an optimum construction of a coil for generating a homogeneous magnetic field from a comprehensive view point as described above.
As mentioned above, various proposals have been provided for an active shield type superconductive MRI magnet, but, no proposal is provided for an optimum construction thereof from a comprehensive view point. Accordingly, when a coil for generating a homogeneous magnetic field is designed and manufactured, the coil is apt to be larger and more expensive than the optimum construction. Furthermore, the active shield type has a disadvantage that a patient may feel more closeness since the overall axial length of the coil for generating a homogeneous magnetic field is further increased due to the provision of an outer side coil.